The present invention relates to acoustic wave devices, such as SAW devices, which are useful e.g. as filters and oscillators, and is more particularly directed to a technique of packaging these devices which is both economical and rugged.
The invention is more specifically with a packaged device which is highly ruggedized to withstand high-G accelerations without failure. The invention is also concerned with a process for packaging these devices in which a plurality of devices are manufactured and packaged as a single wafer, and are not singulated until after packaging, thus avoiding much of the reasons for high costs in fabrication.
The invention particularly concerns an encapsulation technology for ultrasonic devices for electronic signal processing, whose primary mode of wave propagation is surface-related, as in SAW and STW, there by providing improved performance during mechanical vibration and shock, and also allowing an outer enclosure to be formed in place wet, as in polymer molding.
SAW (Surface Acoustic Wave) devices, STW (Surface Transverse Wave) devices, or similar acoustic wave devices are commonly used in various applications, such as in the spectral filtering of electronic signals. The most common type of these are the SAW devices. In the SAW device, any of various techniques are used to launch a Rayleigh wave, and subsequently receive it after it has traveled along a predetermined path along a prepared region of a substrate material, such as quartz crystal, lithium niobate, or lithium tantalate. Usually, there are metal electrodes connected to some metallized termination, using wire bonds. The surface along which the acoustic wave travels is quite sensitive, and this sensitivity becomes extreme as the frequency of operation increases.
The surface also has to satisfy specific boundary conditions for the ultrasonic wave to propagate correctly, and this includes such practical considerations as cleanliness. A Rayleigh wave has an elliptical particle motion as it travels along the surface and is accompanied by an electric field above the surface that penetrates about one wavelength in the direction normal to the surface. Material on the surface attenuates the wave, and this is especially the case for viscous materials or materials having a significant dielectric constant. The optimum environment for this sensitive surface would be a vacuum, but a head space filled with dry neutral gas, e.g., nitrogen, is often employed. The purpose of the head space is to ensure that essentially nothing touches the active surface. This has restricted prior packaging techniques. Also, as the die and the bonding wires are not embedded in any sort of solid material, large or sustained mechanical vibrations or shock can degrade or destroy the SAW device.
Currently, SAW devices are placed in pre-made packages of ceramic or metal. This means that the devices themselves are singulated prior to that time and have to be inserted into their individual packages. A spot of mounting adhesive on the underside holds the SAW in place. In any practical application, a head space of air or nitrogen is required above the upper surface. This head space is required, as aforesaid, to ensure that essentially nothing touches the active surface so that the acoustic waves can travel on that surface. The need for head space limits the ruggedness of the packaging, and limits the mechanical severity of the environment in which the devices may be used. After the head space is filled with gas, an enclosure lid is attached, and is hermetically sealed to the rest of the package.
These packages are typically pre-fabricated by an outside supplier. Because each of the dies has to be installed individually into its own package, the step of packaging can be quite high. Typically, the cost of the finished SAW device is limited by the cost of the package. Also, the degree to which the size of the device can be reduced is also limited by the requirement for head space.